I agree with Post #6; it isn't right, or morally justifiable to steal. However, considering the circumstances, most would steal to save the lives of their children. I also agree with Post #5; Mary should ask for the food.
She could also make a proposal to the store owner that she will perform some kind of work task for the store to 'earn' the food, even if it's a small one-hour task or something that the owner agrees to let her perform, such as some janitorial work. In the end, it's all about compassion and mercy anyways, and getting the children fed is the priority. Addressing the morality of the situation (so stealing doesn't occur again) is something to deal with once food is in the childrens' mouths.
It's not that Mary wants to steal; she believes she has no choice but to steal. Stealing isn't right and the ideal action is for her to do something other than steal. But if she sees no alternatives, she will feel forced to steal the food.